Priority to German Application No. 101 11 871.6, filed Mar. 13, 2001 and hereby incorporated by reference herein, is claimed.
The present invention is directed to a device for recording images on a printing form, including an array of light sources and imaging optics for generating (nxc3x97m) imaging spots on the printing form, n greater than 1 and m greater than =1 being natural numbers.
The time it takes to record an image on printing forms, whether it is in a printing-form imaging unit or in a print unit, is substantially determined by the number of available imaging channels. For that reason, typical image-recording devices have a plurality of light sources, at least one light source being assigned to an imaging channel. By way of an image-recording channel, an imaging spot is projected onto the surface of a printing form, enabling printing dots to be placed on the printing form. Many documents in the technical literature deal with the multi-beam image-recording of printing forms.
U.S. Pat. No. 5,291,329, for example, describes a device for recording images, where a plurality of laser light sources configured in a two-dimensional array are imaged using an optical system onto a recording surface to generate a plurality of imaging spots. The laser light sources are typically configured on a spherical-shell sector such that the main beam of any one laser passes through a focus of a first lens of the optical system.
To reduce the pitch of imaging spots, U.S. Pat. No. 5,995,475 discusses a two-dimensional array of individual multimode laser diodes having separate collimating lenses. The array is imaged at a large reduction ratio onto a recording medium. In this instance, the imaging optics includes anamorphic, optical elements to form the beam of the divergent laser light.
What these image-recording devices have in common is that they use edge-emitting diode lasers to generate the laser radiation. A disadvantageous property of edge-emitting diode lasers is, however, that they exhibit laser radiation having a large angle of divergence and a pronounced astigmatism, making for a complex focusing of the laser radiation and usually requiring an expensive optical lens system. Moreover, edge-emitting diode lasers must first be broken out of a wafer and mounted, prior to performing a functional test. These manufacturing steps are, in part, very expensive; they reduce manufacturing efficiency, and, as a result, increase the price of the lasers.
While conventional semiconductor lasers are edge emitters, thus the light is propagated perpendicularly to the surface of the p-n junction, and the light emerges perpendicularly from the cleavage faces of the chip, so-called surface-emitting laser diodes (VCSEL laser diodes, vertical-cavity surface-emitting lasers) are known, which emit light perpendicularly to the wafer surface. The cavity resonator axis is disposed in parallel to the surface of the p-n junction. In the context of this description and of the device according to the present invention, the concept of a VCSEL light source may embrace all diode lasers, whose beam direction is at right angles to the active zone. These may be, in particular, surface emitters, whose resonator cavity length is short in comparison to the thickness of the active zone, surface emitters, whose resonator cavities are monolithically lengthened, or surface emitters, which have an external or a coupled resonator cavity (also referred to as NECSELs). Furthermore, a VCSEL light source may be a diode laser, whose resonator cavity is situated essentially in parallel to the active zone and is provided with a diffracting or reflecting structure which couples out the laser radiation at right angles to the active zone.
Generally, for VCSEL light sources, it holds that the active length of the resonator can be very short, typically only a few micrometers, and that highly reflecting resonator cavity mirrors are needed to obtain small threshold currents. Surface-emitting laser diodes, VCSEL light sources, have numerous interesting properties. By employing an extremely short resonator cavity, often of a length of less than ten micrometers, one achieves a large longitudinal mode spacing, which promotes single-mode emission above the laser threshold. By using a rotationally symmetric resonator cavity of, typically, six to eight micrometers diameter, one obtains a circular near field andxe2x80x94due to the relatively large diameterxe2x80x94a small beam divergence. Moreover, the geometry of the laser permits a simple monolithic integration of two-dimensional VCSEL laser-diode arrays. Finally, it is possible to test the lasers directly on the wafer following manufacturing.
The typical layer structure of a surface-emitting laser is known to one skilled in the art and can be taken from the relevant literature (see, for example, K. J. Ebeling xe2x80x9cIntegrierte Optoelektronikxe2x80x9d [Integrated Opto-electronics], Springer Publishers, Berlin, 1992). EP Pat. 090 5 835 A1, for example, describes a two-dimensional array of VCSEL light sources which are individually addressable or controllable.
Typical VCSEL light sources have, however, only a modest output power. To increase the attainable output power and to restrict a laser to oscillate in its fundamental mode, U.S. Pat. No. 5,838,715 describes a special resonator cavity form for a VCSEL layer structure. However, the drawback of a procedure of this kind is, inter alia, the expensive manufacturing.
In this connection, it is worth mentioning that it is also known from the literature that light from a plurality of emitter diodes is combined to generate an intensive light beam using imaging optics. For example, U.S. Pat. 5,793,783 describes how light from a plurality of light sources or subarrays of light sources in one array is converged into an overlapping focus.
In addition, it is established from the literature, for example, from U.S. Pat. No. 5,477,259, that an array of light sources can be combined from individual modules of subarrays. Typically, this constitutes a row, thus, one-dimensionally configured laser diodes, which are fixed side-by-side on a mounting element, resulting in a two-dimensional array of light sources.
There are usually two classes of image-recording methods for recording images on a printing form using a one- or two-dimensional array of imaging spots. The first class is based on a dense arrangement of imaging spots of the image-recording light sources. In other words: The spacing among the image-recording points corresponds to the spacing among the printing dots to be set. The two-dimensional printing form to be recorded on is then covered in a translatory movement in the two linearly independent directions defining the surface. The second class of image-recording methods is distinguished by the spacing among the imaging spots being greater than the spacing among the adjacent printing dots to be set. Therefore, a complete recording of images on the two-dimensional printing form requires that the imaging spots of one specific imaging step come to rest between imaging spots of an imaging step that preceded in time. Methods of this kind are also called interleaving methods. U.S. Pat. No. 4,900,130 is cited here as an example of such an interleaving method. This document discusses both a one-dimensional as well as a two-dimensional method for interleaving raster scan lines for a one- and/or two-dimensional array of light sources, whose imaging spots are situated on a recording medium at a larger spatial interval than adjacent printing dots.
An object of the present invention is, therefore, to create a device for recording images on a printing form which is able to record using a multiplicity of image-recording channels and whose light exhibits advantageous beam properties, and/or whose focal points are able to be generated using simple optics. It is additionally or alternatively intended to achieve a long system service life and to render possible inexpensive repairs, in the event of partial failures.
The present invention provides a device for recording images on a printing form (16), including an array (10) of light sources (12) and an imaging optics (14) for generating (nxc3x97m) imaging spots (18) on the printing form (16), n greater than 1 and m greater than =1 being natural numbers. The array (10) of light sources (12) includes an array of (rxc3x97s) VCSEL light sources (12), of which at least two VCSEL light sources (12) are controllable independently of one another, r greater than =n and s greater than =m being natural numbers.
The device according to the present invention for recording images on a printing form thus has an array of light sources and imaging optics for generating (nxc3x97m) imaging spots on the printing form, n greater than l and m greater than =l being natural numbers. A distinguishing feature of the device is that the array of light sources includes an array of (rxc3x97s) VCSEL light sources, of which at least two VCSEL light sources are controllable independently of one another, r greater than =n and s greater than =m and r and s being natural numbers. Surface-emitting diode lasers (VCSEL) have advantageous beam properties. Because of the extended emitter surface, the radiation is emitted at a small angle of divergence. The beam quality and the shape of the emitted beam are substantially determined by the size of the outcoupling facet. Through selection of a specific size, a VCSEL generates radiation in the fundamental mode of the resonator cavity (Gaussian beam), which is especially advantageous for recording images on a printing plate due to the substantial depth of focus. In contrast to edge-emitting lasers, beam diameters and divergence angles are the same in the linearly independent directions defining the beam diameter, so that a collimation and a focusing may be achieved using relatively simple optical elements (fewer asymmetrical and/or aspherical elements), in the form of micro-lens arrays as well. A suitable contacting of the individual VCSELs in the array ensures that the individual lasers are able to be controlled separately and independently of one another, making it possible to vary the light intensity in the image-recording channels assigned to the VCSEL light sources.
In other words, an important idea of the present invention is to employ a VCSEL array to record images on printing forms, where, for each image-recording channel, a high enough power output is generated by VCSEL emitters in an advantageous resonator mode. Using an individually controllable array, in each image-recording channel, an intensity corresponding to the image-recording information may be impressed on the imaging spot. To attain a higher power output for each image-recording channel, it may be advantageous to excite other modes, apart from the fundamental mode of the resonator cavity.
In particular, to increase the available power, the device for recording images on a printing form may be designed such that at least one imaging spot is formed on the printing form by combining the light emitted by a subarray of the (rxc3x97s) VCSEL light sources, thus, at least two VCSEL light sources. By allocating in this manner a plurality of VCSEL light is sources to one image-recording channel, one advantageously increases the redundancy and the system service life, at the same time. Should one VCSEL light source fail, others are still available within the image-recording channel.
It is especially beneficial to provide the device for recording images on a printing form with an array of VCSEL light sources which is constructed in modular fashion from a number of subarrays. In other words, the total array of the (rxc3x97s) VCSEL light sources may be fabricated from individual strips or blocks, each including a plurality of VCSEL light sources, for example s blocks having r surface emitters. In the event of a partial failure of certain light sources, these may be replaced quickly, simply, and inexpensively.
In one alternative specific embodiment of the device according to the present invention, which produces oval foci on a printing form, preferably with an ellipticity of at least 1%, whether it be due to an elliptical geometry of the surface emitters or due to suitable optics, a higher energy density may be achieved in relation to the moving direction using a transverse-elliptical (oblate) beam and a more favorable coupling of the energy into the printing form using a longitudinal-elliptical (prolate) beam than when a round beam is employed, which generates the same line width.
VCSEL arrays count among their properties that the substrate edges are exposed and do not perform the function of emitting facets as in the case of edge-emitting diode lasers. This makes it possible to process the edges very precisely, enabling individual arrays to be positioned relatively to one another. As a result, a plurality of one- or two-dimensional VCSEL-arrays may be positioned with respect to a larger array, thus subarrays may be positioned with respect to an (rxc3x97s) array of VCSEL light sources, where the regularity of the geometric arrangement of the emitters extends over the entire array. Therefore, a modular-type construction makes it possible to achieve very large arrays, a failure of individual emitters on one module being easily rectified by replacing only the affected module.
An efficient cooling is rendered possible when, in one advantageous specific embodiment of the device of the present invention for recording images, the VCSEL is mounted with the p-side facing downwards (on a carrier or a holding element).
By configuring the VCSEL light sources two-dimensionally, one obtains a lower thermal resistance than in the case of an array of edge-emitting diode lasers, due to a more homogeneous, approximately one-dimensional heat flow. In the case of edge-emitting diode lasers, the contact surface between regions, in which heat is produced, and the heat sink(s) typically amounts to 0.1 cm2; in the case of a VCSEL array, however, typically to 1.0 cm2.
In this connection, it should be mentioned that, in contrast to edge-emitting diode lasers, VCSELs exhibit a quite readily reproducible ageing behavior. Spontaneous failures, for example due to facet destruction, are relatively rare. This signifies a prolonged overall service life for the VCSEL arrays, since, in all probability, no emitters fail until after an extended period of time; this contrasts to the facet destruction of an edge emitter, which occurs approximately at a specific rate, regardless of ageing. The reproducible ageing may advantageously be utilized in that the power of a reference emitter is measured, so that the power loss caused by ageing of the reference emitter and of an assigned group of emitters may be compensated without having to determine or measure the power of each individual emitter.
Besides the mentioned scalability of a preferably individually controllable VCSEL array, it is significant that the inherent system properties of VCSEL arrays facilitate a device which is more cost-effective than a device which utilizes edge-emitting laser diodes. Moreover, the geometric dimensions of a VCSEL array are considerably smaller, so that a very compact image-recording device may be created for printing forms.
By incoherently focusing the light from at least two VCSEL light source in an image-recording channel, at the same point, the VCSEL light sources in the subarray assigned to the image-recording channel may be beneficially controlled such that the light emitted by a VCSEL light source exhibits a fixed phase relation to the light emitted by a second VCSEL light source. Utilizing constructive interference effects, a beam may be formed which possesses a high power output, in connection with a high beam quality. Therefore, in this case, producing a specific imaging spot requires driving the VCSEL light sources in question simultaneously. In other words, a plurality of emitters, thus VCSEL light sources, are combined into one image-recording channel.
The device according to the present invention for recording images on a printing form maybe utilized to special advantage in a printing-form imaging unit or in a print unit. In this context, it may concern both flat-bed imaging units as well as printing forms accommodated on curved holding elements, for example on a cylinder. The two-dimensional surface of the printing form is swept over by the imaging spots of the device for recording images, in a fast and slow feed direction (linearly independent directions, not necessarily at right angles to one another). A printing press in accordance with the present invention, which includes at least one feeder, a print unit, and a delivery unit (sheet-fed press), has at least one print unit including a device according to the present invention for recording images on a printing form. Alternatively thereto, a web-processing printing press may also have a device according to the present invention. The printing press is preferably a direct or indirect flat-bed printing press, in particular an offset press.
In this context, a two-dimensional configuration of the array may advantageously be such that the r columns (having s VCSEL light sources) are disposed substantially perpendicularly to the fast feed direction, thus the s lines of the array are disposed essentially perpendicularly to the slow feed direction. When q is the distance between two adjacent surface emitters, and d is the distance between two lines on the printing form, then, preferably, s=q/d. To produce x-times redundancy, preferably s=x*q/d. A low sensitivity to tilting of the array and time-related control errors may be achieved by a geometric configuration, in which the maximum distance, in the fast feed direction, between two VCSEL light sources, which record adjacent lines, is less than s*q.
By using a very large array, i.e., one that is modularly assembled out of subarrays, a 1:1 image recording is possible. In other words, the entire surface of the printing form on which images of a specific number of printing dots are to be recorded is exposed using the same number of image-recording channels. This may be beneficial, particularly when working with flat-bed exposure, when the aim is to achieve a very fast printing form imaging unit. Due to the advantageous beam properties of VCSEL light sources, a relatively simple imaging optics may be used, or, however, in the case of a very small image-recording distance, a direct image recording may take place without imaging optics.
It is worth mentioning that the printing form may be both a conventional printing form, thus, for example, a customary thermal printing form, as well as a re-recordable printing form, a printing master or a film. When working with a digitally recordable printing form, whose sensitivity to a specific wavelength of the image-recording light becomes extreme, VCSEL light sources having corresponding emission wavelengths must be used. In certain applications, it may be advantageous not to use the VCSEL light sources in the continuous wave mode, thus for emitting continuous radiation, but rather for generating short pulsed radiation.